drm/xe/vm: move xa_alloc to prevent UAF

drm/xe/vm: move xa_alloc to prevent UAF

Evil user can guess the next id of the vm before the ioctl completes and
then call vm destroy ioctl to trigger UAF since create ioctl is still
referencing the same vm. Move the xa_alloc all the way to the end to
prevent this.

v2:
 - Rebase

Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Signed-off-by: Matthew Auld <[email protected]>
Cc: Matthew Brost <[email protected]>
Cc: <[email protected]> # v6.8+
Reviewed-by: Nirmoy Das <[email protected]>
Reviewed-by: Matthew Brost <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]